The present invention relates to a heat engine, in particular for low-temperature operation for the utilisation of solar heat, waste heat from biological or industrial processes or the like, with at least two cylinder-piston units, each containing an expansion fluid, which stands under prestressing pressure and which changes its volume in the case of a change of temperature and thus moves the piston, elements for the individually controllable supply of heat to the expansion fluid of each cylinder-piston unit, and a control means controlling the heat supply elements to allow each expansion fluid to alternately heat up and cool down and thus move the pistons.
Such a heat engine is known from U.S. Pat. No. 5,916,140. Effective expansion fluids frequently require a specific prestressing pressure in order to show a significant coefficient of expansion in the desired operating temperature range. An example of this is liquid carbon dioxide, which changes its volume by about 2.2-fold when heated from 20° C. to 30° C. at a pressure of approx. 60-70 bar.
U.S. Pat. No. 5,916,140 discloses different variants to place the expansion fluid in the cylinder-piston units under the required prestressing pressure. On the one hand, metal or gas springs are proposed to prestressing the pistons in the direction of the expansion fluid. However, no prestressing pressure independent of the piston movement can be reached with such a distance-dependent spring force. On the other hand, a mechanical coupling of two cylinder-piston units by means of a crankshaft or by opposed cylinder assembly is described, so that the respectively extending piston maintains the prestressing pressure on the expansion fluid of the retracting piston. However, such a rigid coupling requires that the heating and cooling phases are about equal in length, since otherwise a piston that retracts too slowly will hinder the extending one, which is detrimental to efficiency, or a piston that extends too slowly will generate too little prestressing pressure to assure operation.
It is proposed in U.S. Pat. No. 5,916,140 as a solution to the last-mentioned problem to accelerate the cooling phase by removing heat as rapidly as possible, so that it is always shorter than the heating phase. However, this is scarcely realisable in practice, since highly variable heat supply is to be expected especially when using solar heat. Thus, for heating liquid carbon dioxide from 20° C. to 30° C. at midday, for example, a heat supply temperature of 70° C. and thus a temperature difference of 40-50° C. may be available, whereas for cooling from 30° C. to 20° C. there is only a temperature difference of 15-25° C.—even in the case of forced cooling with cold water at 5° C.—, and as a result a cooling phase that is about double the length of the heating phase is to be expected. On the other hand, in the morning and evening hours the temperature level of the solar plant may also only amount to 30°-40° C., for example, and as a result of which a heating phase that is longer than the cooling phase may even be expected.